I'd check the man page, but since I just rewrote it I don't think that's
going to help!

I have two nodes, A and B, and I run "mpirun -hostfile myhostfile
-tag-output hostname" with five different hostfiles. Here is what I get:

B slots=2
B slots=2
A slots=2
A slots=2
B B B B A A A A

B slots=2
A slots=2
B slots=2
A slots=2
B B B B A A A A

A slots=2
B slots=2
A slots=2
B slots=2
A A A A B B B B

B slots=2
A slots=2
A slots=2
B slots=2
A A A A B B B B

A slots=1
B slots=1
A slots=1
B slots=1
A slots=1
B slots=1
A slots=1
B slots=1
A A A A B B B B

This is with openmpi-1.7a1r22109. After each hostfile, I list the nodes
I get in MPI rank order. So, it appears the hostfile cannot be used to
control in what order processes are mapped to nodes. It can only be
used to specify the total number of slots per node. Slots are filled up
one node at a time. I assume we don't want to make any claims about the
node order?